1. Field of the Invention
The present invention relates to extrusion dies for the production of ceramic honeycomb structures to be used as catalyst carriers, filters, heat exchangers and the like for the purification of exhaust gases.
2. Related Art Statement
Ceramic honeycomb structural bodies are now used as catalyst carriers for the purification of exhaust gases from internal combustion engines, filters for the removal of soot and dusts in exhaust gases from diesel engines and rotary type heat exchangers.
Japanese Patent Application Laid-open No. 50-75,611 (corresponding to U.S. Pat. No. 3,885,977) discloses a process for producing such ceramic honeycomb structural bodies, in which cordierite is formed by extrusion. Japanese Patent Publication Nos. 55-41,908 (corresponding to U.S. Pat. No. 3,790,654) and 57-61,592 (corresponding to U.S. Pat. No. 3,905,743) disclose extrusion dies 1 as shown in FIGS. 4(A) and 4(B), in which ceramic material feed holes 2 are opened in one surface for receiving a ceramic material through an extrusion machine, and forming channels 3 are opened in the other surface corresponding to a sectional matrix of a ceramic honeycomb structural body, while intersecting sections 4 are provided between the feed holes 2 and the forming channels 3.
Although not shown, U.S. Pat. No. 3,308,201 disclose dies in which stagnating portions are formed between ceramic material feed holes and forming channels for temporarily stagnating the body.
Furthermore, Japanese Patent Publication No. 61-39,167 discloses a technique for producing extrusion dies, in which forming channels are worked by machining and/or by discharging, and then electrolessly plated to form forming channels having a given width.
In addition, in order to prolong use life of extrusion dies, dies are known in which an electrolessly plated composite layer consisting of electroless plating and wear resistive grains is deposited on surfaces of ceramic material feed holes and forming channels [Japanese Patent Application Laid-open No. 63-176,107 (corresponding to U.S. Pat. No. 4,861,626)], and dies are also known in which a wear resistive material is chemically vapor deposited thereon. [Japanese patent application Laid-open Nos. 60-145,804 (corresponding to U.S. Pat. No. 4,574,459) and 61-69,968].
However, the conventional processes for producing ceramic honeycomb structural bodies have the following drawbacks.
Although the conventional processes have a merit that honeycomb structural bodies having thin walls can be mass produced, it often occurs that the conventional dies frequently cause honeycomb structural bodies to deform or to suffer troubles such as warpage during extrusion, or deformation or cracking after firing at the starting of use of such dies. Under the circumstances, the present inventors have examined causes therefor, and discovered that these troubles are due to non-uniform dimensional accuracy and surface roughness of the ceramic material feed holes or forming channels of the die.
That is, the honeycomb structure-extrusion die has a number of ceramic material feed holes and forming channels, and their dimensions are very small. Further, the depth of the material feed holes is deeper as compared with the inner diameter thereof. For instance, when a honeycomb structural body having a cell density of 400 cells/inch.sup.2, a square cell sectional shape, an outer diameter of 180 mm and a wall thickness of 0.15 mm is to be obtained by extrusion, it is necessary that the inner diameter and the depth of the material feed holes are 1.3 mm and 17 mm, respectively, and the number of feed holes is about 3,400. Further, it is necessary that the width of the forming channels is about 0.17 mm and the forming channels are provided at the number of about 100 in each of transverse and longitudinal directions.
Therefore, it is difficult to uniformly machine the material feed holes and the forming channels. As schematically shown in FIG. 5, the surface roughness and the straightness of the material feed holes 2 vary. When the ceramic material is extruded by using the thus machined die, the flow resistance of the ceramic material varies depending upon the material flow paths of the die. Thus, a honeycomb structural material having a uniform face in a direction orthogonal to the extruding direction cannot be extruded. Rather, the shape of a head of the extruded structure 9 is nonuniform as shown in FIG. 6. Even if the thus extruded material is not cracked or greatly deformed during the extrusion, a desired dimension cannot be obtained due to residual stresses inside the extruded structure or the walls of the cells in the honeycomb structure are cracked due to the residual stresses, when the extruded structure is dried and fired.
In order to solve the problems mentioned above, the surface roughness and the dimensional accuracy of the material feed holes and the forming channels can be made uniform by honing or reaming the surfaces thereof, but this unfavorably increases the number of working steps. On the other hand, if the ratio of the depth of the material feed holes to the diameter thereof is great, the surface roughness becomes more non-uniform, which makes the above honing or reaming difficult. In this case, a bonding-die technique in which a forming section and a feed hole section are separately machined and bonded together, is available. However, this technique suffers from an excessive number of working steps.
Further, another conventional technique is also available, in which the dimension of the material feed holes or the forming channels is adjusted by electrolessly plating them to prolong use life. However, the present inventors recognized that the speed at which the honeycomb structure is extruded through the forming channels more or less differs in the above case, and that the differences in the extruding speed cause the troubles.